Rotate alphabet characters in a string by an arbitrary amount

rot_n_string.js

//needed to make friendly for map function, which expects function taking dereferenced string index as argument
function charCodeOfChar(character){
  return character.charCodeAt(0);
}

//String's prototype function will give weird UTF-16 literal output otherwise
function charFromCharCode(charCode){
  return String.fromCharCode(charCode);
}

//shifter for individual characters
function rotXShifter(asciiCode, shift) {
  shift = shift > 0? shift : (shift%26) + 26;
  if (asciiCode > 64 && asciiCode < 91)
    return ((asciiCode - 65 + shift) % 26) + 65;
  if (asciiCode > 96 && asciiCode < 123)
    return ((asciiCode - 97 + shift) % 26) + 97;
  return asciiCode
}

//shifter for complete strings, ignores non-ASCII alphabetical characters
function stringShifter(string, displacement = 13){
  return string.split('')
    .map(charCodeOfChar)
    .map( x => rotXShifter(x, displacement) )
    .map(charFromCharCode)
    .join('');
}

console.log(stringShifter('Victory!'));